Save another cookie when a particular user has already filled his/her form.
You can create a cookie key by using username. Something like this:

var cookie_FormFilled = username + "_form_filled";

Now when user comes at the page, get its username and build above cookie key. For example username is 'xyz', then the key will be 'xyz_form_filled'. Read the this cookie if you get 'true', then user has already filled his/her form. If the values comes out to be null/undefined then prompt the user with the form. And once form is filled save the cookie with value 'true'.

I am sorry. I appreciate the insigt into this but this is still just greek to me. When I say that I have no experience with cookies, I mean no experience.

What you say probably makes sense to someone that has at least used a cookie before but too me, I just do not get it and the more I try to get my head around the cookie the more confused I am getting. This is frustrating becuase I understand HTML and JavaScript.